1. Field of the Invention
The instant invention relates to an electrohydraulic flow control valve in which the size of a metering orifice is automatically adjusted in response to the amount of current supplied to a solenoid incorporated in the device. The flow control valve can be used in conjunction with a pressure compensator, which will assure a constant pressure differential across the orifice regardless of variations in pressure upstream and downstream of the orifice, to provide a constant flow.
2. Description of the Prior Art
A simple form of flow control is a fixed orifice. A more sophisticated type of flow control is a manually adjustable, variable orifice. One type of manually adjustable, variable orifice is a needle valve.
An orifice, either fixed or adjustable, is inserted in a line and sets the area available for fluid flow. It is frequently used with a pressure compensator to provide a constant flow of fluid to a device. A pressure compensator maintains a constant fluid pressure differential across the inlet and outlet of the orifice. If the pressure drop across the orifice is uniform, the flow through the orifice is constant regardless of variations in fluid pressure upstream or downstream of the orifice.
One device which requires a constant fluid flow rate is a hydraulic motor. This enables the motor to turn at a constant speed. Another device is a hydraulic cylinder where it is necessary to provide a constant flow of fluid to enable the piston to move at a constant speed in the cylinder.
U.S. Pat. No. 3,246,669 to Adams and assigned to the assignee of the instant invention discloses a fixed orifice, pressure compensated flow control. This control uses pilot fluid for a control fluid. The control fluid is fed from the inlet port through a fixed orifice and is pressure compensated. The pressure drop across the fixed orifice is maintained constant by a control piston in which the right end of the piston is exposed to pressure upstream of the orifice and the opposite end is subjected to fluid pressure downstream of the orifice which acts in conjunction with a light spring to resist movement of the piston caused by the high pressure fluid in a well known manner. Movement of the control piston causes a land on the control piston to restrict or increase flow through a port to thereby maintain a constant flow of fluid through the orifice.
A more sophisticated flow control is shown in U.S. Pat. No. 3,200,832 to Adams and assigned to the assignee of the instant invention. This control uses an electrical device for changing the size of a metering orifice. In this control the pressure differential of pilot fluid across a variable orifice acts on a metering spool to change the size of the metering orifice. The maximum force available to move the metering spool is limited to the amount of pressure drop across the variable orifice which is typically 100 psi. This control includes a pressure compensator assembly which assures a constant pressure differential across the main metering orifice.
An electrohydraulic flow control where the pilot fluid is pressure compensated to assure a constant flow rate is also shown in U.S. Pat. No. 3,159,178 issued to Adams and assigned to the assignee of the instant invention. With a constant rate of fluid flow, the gap between the armature and the core remains the same for any selected flow rate setting and therefore the force in the poppet which controls the level of pilot pressure remains constant. If the pilot flow were not constant and the gap between the armature and core varied to accommodate different flow rates, the electro-magnetic force acting on the piston which controls the pilot pressure would not be uniform and the opening of the controlled orifice would not correspond to a given setting (either current or voltage) of the electric controller.
Another type of electrically controlled orifice can be found in a jet pipe or flapper type servo valve. In this type of valve the metering orifice spool is caused to move by the pressure differential across the ends of the spool caused by the difference of pressure between the jet pipe receiver ports. The difference in pressure in these receiver ports is a maximum of approximately 65% of the fluid inlet pressure at maximum input to the jet pipe coil.
It has been found that it is sometimes necessary to have a larger pressure differential acting across the spool which sets the size of an orifice than can be obtained from a conventional flow control using a pilot fluid or a jet pipe type of servo valve.